1. Field of the Invention
The present invention relates to a method for controlling a fuel cell system.
2. Discussion of the Background
A fuel cell system acquires DC electric energy according to an electrochemical reaction of a fuel gas (gas essentially containing hydrogen, such as hydrogen gas) and an oxide gas (gas essentially containing oxygen, such as air) respectively supplied to an anode electrode and a cathode electrode. This system is of a stationary type, or is mounted in a fuel cell vehicle as an on-vehicle fuel cell system.
For example, a solid polymer fuel cell has an electrolyte membrane/electrode assembly (MEA) having an anode electrode and a cathode electrode provided on the respective side of an electrolyte membrane formed by a polymer ion-exchange film; the electrolyte membrane/electrode assembly is sandwiched by a pair of separators. A fuel gas passage for supplying a fuel gas to the anode electrode is formed between one of the separators and the electrolyte membrane/electrode assembly. An oxide gas passage for supplying an oxide gas to the cathode electrode is formed between the other separator and the electrolyte membrane/electrode assembly.
When the fuel cell is stopped, supply of the fuel gas and oxide gas is stopped. However, the fuel gas remains in the fuel gas passage, and the oxide gas remains in the oxide gas passage. When the operation-stop period of the fuel cell becomes long, therefore, the fuel gas and the oxide gas may pass through the electrolyte membrane, so that the fuel gas is mixed with the oxide gas to react therewith, thereby deteriorating the electrolyte membrane/electrode assembly.
To cope with the problem, when the operation of the fuel cell is stopped, a fuel cell system disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-22487 (FIG. 1 and paragraph [0029]) executes what is called an in-stop-mode power generating process of shutting off the supply of a reaction gas to the anode side, and reducing the supply of the reaction gas to the cathode side to maintain power generation, thereby charging a battery with the generated power. The in-stop-mode power generating process causes hydrogen to be consumed on the anode side, and causes oxygen to be consumed on the cathode side, thereby filling the cathode side with a nitrogen gas. This suppresses deterioration of the fuel cell system at the time of soaking after the operation of the fuel cell is stopped.